EP0370909A1 - Process for the conversion of natural gas or of light alkanes into unsaturated hydrocarbons - Google Patents

Abstract

The present invention relates to a process for conversion of natural gas or of a light alkane or light alkanes into unsaturated hydrocarbons. <??>According to the invention, a fluidised bed of particles of a refractory and advantageously catalytic material is created inside a reaction space, and a plasma of a gas containing hydrogen and the natural gas or the light alkane or alkanes is introduced into the bed in such a way that the latter quenches the reaction medium and catalyses the conversion reaction. <??>The invention is in particular applicable in the chemical and energy-producing industries.

Description

The present invention relates to a process for converting natural gas or light alkane (s) into unsaturated hydrocarbons and more particularly to a conversion process with supply of electrical energy applied in particular in the chemical and energy industries.

There are currently conversion processes with the supply of electrical energy. A representative process is the Hüls process which allows the conversion of hydrocarbons such as methane to unsaturated hydrocarbons and in particular to acetylene. This process involves passing methane through an electric arc and then separating the products obtained. However, this process has the disadvantage of forming a very large amount of carbon black.

Also, an improvement has been proposed using a hydrogen plasma. The hydrogen plasma provides the energy necessary for the methane conversion reaction, which is an endothermic reaction. This energy is brought in situ by the plasma gases without the intermediary of a wall. However, if the plasma is a source of energy in the methane conversion reaction, it still has disadvantages because its temperature is too high for the envisaged reaction. In fact, the methane brought to a temperature above 1200 ° C. is broken down by a succession of dehydrogenation and cyclization reactions into a mixture of polyaromatic substances leading to carbon black.

These methods are therefore not completely satisfactory since they involve too much black formation. of uncontrolled quality carbon which therefore becomes a difficult-to-recover by-product.

The object of the invention is a method which does not present the difficulties and drawbacks of the known methods.

To achieve this object, the invention is characterized in that a fluidized bed of particles of refractory and advantageously catalytic material is created inside a reaction space and that a plasma is introduced of a gas containing hydrogen and natural gas or the light alkane (s) in the bed so that it quench the reaction medium and catalyze the conversion reaction.

According to a characteristic of the invention, the bed of particles is fluidized by a gas stream of fluidization advantageously containing hydrogen.

According to yet another characteristic of the invention, the gaseous fluidization stream contains hydrogen and argon.

According to a preferred embodiment of the invention, the natural gas or the light alkane (s) is introduced into the bed with the gaseous fluidization stream and the light alkane is methane. The fluidizing hydrogen and the methane are introduced into the bed in a hydrogen / methane proportion ranging from 0.5 to 10 and preferably from 2 to 5.

According to another characteristic of the invention, the gaseous fluidization stream is preheated upstream of the bed to a temperature between 50 ° C and 500 ° C and preferably between 150 ° C and 350 ° C.

According to a feature of the process of the invention, a plasma is introduced containing at least 10% hydrogen and which may contain argon.

According to another feature of the invention, the bed consists of particles of a material chosen in particular from the group consisting of oxides, carbides, nitrides and borides.

According to another feature of the invention, the particles produce a catalytic effect.

According to yet another feature of the invention, the bed also contains a catalyst.

The conversion reaction is carried out in the bed at a temperature between 500 ° C and 1200 ° C and preferably between 500 ° C and 800 ° C.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the single figure representing a diagram of a preferred embodiment. of the invention.

The method of the invention is implemented using a device of the type shown in the appended figure and comprising an enclosure 1 comprising at its bottom means 2 for injecting a gaseous stream of fluidization, means for leaving the latter (not shown) and containing a mass of particles of a material intended to form a fluidized bed 3, and a plasma torch 6 of a gas containing hydrogen, suitable for introducing the plasma inside the enclosure in the fluidized particle bed.

There is an evacuation tube 5 connected to the outlet of the enclosure.

The plasma torch is connected to a wall of the enclosure so that the plasma is introduced into the fluidized bed. The angle of introduction of the torch into the enclosure can be varied from 0 ° to 90 °. Preferably, the angle of introduction of the torch is 20 ° relative to the horizontal section of the enclosure. Typically this torch consists of two concentric tubes of silica, with an external diameter of 30 mm, surrounded by five hollow inductive turns of copper cooled by water, traversed by an electric current of high frequency.

The internal wall of the enclosure 1 is for example made of refractory alumina 4 mm thick, insulated by a layer of 20 mm of porous bricks. The whole is covered with glass wool and an asbestos ribbon. The dimensions of the enclosure larger than those of the plasma avoid direct contact of the walls with the hot zone of the plasma. The enclosure has a pyramidal zone in which the particles are suspended and thermocouples (not shown) are installed in the enclosure to measure the temperature of the fluidized bed. A hemispherical lapping (not shown) installed on one of the sides of the enclosure, allows the introduction of the plasma thus treating all the particles.

The means 2 for injecting the gaseous fluidization stream comprise for example a 40 mm opaque silica tube surrounded by a heating tape and filled with refractory balls, this system allows the preheating of the gases and a thermocouple (not shown) is provided. in the tube to control the temperature of the fluidizing gases.

The evacuation tube 5 is for example constituted by a quartz tube 85 mm in diameter and 500 mm in length and thermocouples (not shown) are installed in this tube to measure the temperature of the gas stream passing through it. The outlet of this tube can be connected to a water heat exchanger (not shown) in which the reaction mixture is cooled before being taken for analysis.

The bed consists of particles of a material chosen in particular from the group consisting of oxides, carbides, nitrides, and borides. The following list can be used as examples:
- oxides
aluminum Al₂O₃
magnesium MgO
calcium CaO
BeO beryllium
CeO cerium
ThO₂ thorium
hafnium HfO₂
by lanthanum La₂O₃
and other mixed oxides
- carbides
silicon SiC
of thorium ThC
boron B₄C
- nitrides
boron BN
Hfniumm HfN
ZrN zirconium
- borides
ThB₄ thorium
NobB₂ niobium
ZrB₂ zirconium
- carbon (graphite) C

Whatever the nature of the materials used, these must be refractory because the particles of the bed must be able to withstand high temperatures since they are in contact with the plasma jet. The particles in the bed can themselves act as a catalyst and it is also possible to add another catalyst to them.

It should be understood that the word "catalyst" is taken in its broad sense, that is to say that the particles can accelerate certain desired reactions or inhibit certain undesired reactions such as the formation of carbon black or coke.

In the process of the present invention, the particles of the bed are made to fluidize into a gushing bed by the flow of a gaseous stream of fluidization formed mainly of hydrogen or of a mixture of hydrogen and argon introduced into the 'enclosure 1 using the injection means 2 and the natural gas or light alkane (s) to be converted are introduced into the thus fluidized bed. Preferably, as shown in the device of the figure, the natural gas or the light alkane (s) is introduced into the fluidized bed with the gaseous fluidization stream. The optimum amount of fluidizing hydrogen is determined so as to minimize the formation of carbon black. The fluidizing gases are preheated upstream of the bed, in the tube 2, to a temperature between 50 ° C and 500 ° C and preferably between 150 ° C and 350 ° C.

The plasma torch 6 injects a hydrogen plasma, which may contain argon and containing at least 10% hydrogen, into the fluidized particle bed where a homogeneous transfer of heat takes place between the plasma and fluidized bed , thus making it possible to carry out a conversion reaction in the presence of radical hydrogen at an adjusted temperature which remains notably lower than that of plasma, which therefore minimizes the formation of carbon black. The unsaturated hydrocarbons obtained by the reaction of conversion carried out inside the fluidized bed are then evacuated by tube 5. Thermocouples installed in this tube make it possible to measure the temperatures.

The use of a fluidized bed in the process according to the invention has significant advantages for the following reasons:
- its heat transfer properties allow efficient quenching of the plasma;
- Its viscosity substantially equal to that of the plasma ensures a very good mixture between it and the fluidized bed; and
- its possible catalytic properties can ensure the direct transformation of the product or products to be converted into unsaturated hydrocarbons.

On the other hand, the nature of the material of the particles constituting the bed and / or the nature of the catalyst making it possible to direct the conversion towards the desired products.

The following eight examples are given in order to clearly demonstrate the advantages of the present invention.

In general, the examples were carried out as follows:

The plasma torch operates at a frequency of 5 MHz for a power of 4.4 kW. The injection angle is 20 °. The plasma gases introduced are argon, at a flow rate of 30 l / min and hydrogen at a flow rate of 5 l / min. The bed is made of alumina particles (650g) of 300 microns in average diameter. The particles of the bed are put in fluidization by a mixture of methane, hydrogen and argon. By adjusting the flow rate of these three gases, the residence time of the methane in the reactor is adjusted. Good fluidization is obtained for a total flow of between 15 l / min and 40 l / min. The optimal amount of hydrogen fluidization is determined relative to that of methane so as to minimize the formation of carbon black. This ratio is between 0.5 and 10 and preferably between 2 and 5. The fluidization gases are preheated to a temperature between 50 and 500 ° C, preferably between 150 ° C and 350 ° C. The plasma gas and fluidization flow rates are measured and regulated using mass flow meters. Thermocouples are installed to measure the temperature of the fluidizing gases upstream of the enclosure, the wall of the enclosure, the fluidized bed and the temperatures in the tube 5.

The temperature of the fluidized bed is chosen as the reference temperature since on the one hand it characterizes the efficiency of the quenching and on the other hand because the conversion reaction takes place in the fluidized bed.

The analysis of the products is carried out by gas chromatography.

The eight examples are detailed in Tables 1 and 2 below. They were carried out with the same mass of identical particles and with identical operating conditions of the torch. They differ from each other by the flow rates of the fluidizing gases and by the average temperature of the fluidized bed. The results obtained with these different examples are also detailed in Tables 1 and 2 below. TABLE 1 OPERATING CONDITIONS AND ANALYSIS RESULTS No. Ex. Average temperature in the fluidized bed ° C Fluidization gas (l / min) Molar composition of conversion products (%) Ar H2 CH4 (C2H2) (C2H4) (C2H5) (VS) 1 580 11 8 5 11 2.5 0.5 11.5 2 700 11 8 5 9 2 0.4 17 3 770 11 8 5 6 2 0.4 30 4 500 15 8 5 1.5 0.3 0.1 5 5 600 6 8 5 4 1.5 0.3 22 6 600 0 8 5 7 2.5 28 7 550 0 13 5 21 13.5 2 5 8 * 580 11 8 5 0.2 0 0 0.3 * torch stopped (C) = carbon black concentration RESULTS No. Ex. CH4% conversion Selectivity C2% Efficiency C2% 1 34 71 24 2 35 57 20 3 43 40 17 4 9 42 3.8 5 32 34 11 6 43 38 16 7 57 94 53 8 * 0.7 57 4 * torch stopped

The results obtained and listed in Tables 1 and 2 show the advantage of the process and particularly prove the efficiency of quenching by the fluidized bed since the average temperature in it is relatively low (between 500 ° C and 800 ° C).

Examples 1 to 3 were carried out under identical fluidization conditions but at three different temperatures. The results show that the amount of carbon black formed increases very rapidly with temperature. It emerges from these results that temperature control is essential and we therefore understand the advantage of soaking the plasma.

Example 4 was carried out at a temperature of 500 ° C. Note that the conversion rate of CH₄ at this temperature is only 9%. Consequently a temperature of about 500 ° C constitutes the lower limit for the conversion of methane.

Examples 5 and 6 were carried out at the same temperature but with a different fluidization flow. There is an increase in the conversion rate when the flow rate of the fluidizing gas decreases, that is to say when the residence time of the methane increases. The control of this important parameter can therefore be done easily.

If we compare Examples 6 and 7, we note a clear increase in the yield of C2, certainly due to the increase in the hydrogen / methane ratio. The role of hydrogen fluidization is manifold. In particular it inhibits the dehydrogenation reactions leading to carbon black and it "protects" the methane against thermal shocks, but it can also limit the conversion reaction, hence the need to determine its optimal concentration.

Example 8 was carried out to observe the specific role of plasma which is to provide in high concentration radical species. To do this, a simple experiment was carried out which consists in stopping the torch and immediately analyzing the reaction mixture. Note that at the same temperature but without the plasma, the methane conversion rate is negligible.

Although these examples were obtained without optimization of the operating parameters, it will be noted the very good results of Example 7 in which the selectivity for C2 (acetylene) is 94%, that is to say already better than that of the process Hüls which is around 74%.

avec (X) concentration molaire du constituant X dans le mélange réactionnel, donnée par l'analyse chromatographique.Methane conversion is defined by the ratio of the amount of methane converted to the total amount of methane introduced. It is calculated as follows:

with (X) molar concentration of component X in the reaction mixture, given by chromatographic analysis.

C2 selectivity is the ratio of the quantity of C2 products obtained by the quantity of conversion products. It is calculated as follows:

The yield in C2 is defined by the ratio of the quantity of C2 products obtained by the quantity of methane introduced. It is calculated as follows

Of course, the invention is in no way limited to the embodiments described and illustrated which are given only by way of example. Thus, depending on the nature of the catalyst, the charge and the operating conditions, it is possible to obtain hydrocarbons greater than C₂. Furthermore, the products to be converted could be introduced into the fluidized bed, differently from the example shown, that is to say separately from the fluidization gas, at any suitable location provided that the quenching of the plasma by the bed is respected. fluidized. It is also understood that the plasma used can be produced in any manner, in particular by blown or transferred electric arc or even by induction.

Claims (13)

1. Method for converting natural gas or light alkane (s) into unsaturated hydrocarbons in which a natural gas is brought into contact with a plasma of a gas containing hydrogen in a reaction space and recovers said unsaturated hydrocarbons at an outlet from said reaction space, characterized in that a fluidized bed of particles of refractory and advantageously catalytic material is created inside said space, and that said plasma and said are introduced natural gas or said light alkane (s) in said bed so that the latter quench the reaction medium and catalyze the conversion reaction. 2. Method according to claim 1, characterized in that said bed of particles is fluidized by a gas stream of fluidization advantageously containing hydrogen. 3. Method according to claim 2, characterized in that said gaseous fluidization stream contains hydrogen and argon. 4. Method according to one of claims 1, 2 or 3, characterized in that said natural gas or said light alkane (s) is introduced into the bed with the gaseous fluidization stream. 5. Method according to one of claims 1 or 4, characterized in that the light alkane is methane. 6. Method according to one of claims 2, 3, 4 or 5, characterized in that hydrogen and methane are introduced in a hydrogen / methane proportion ranging from 0.5 to 10 and preferably from 2 to 5. 7. Method according to one of claims 2 to 6, characterized in that the gaseous fluidization stream is preheated to a temperature between 50 ° and 500 ° C and preferably between 150 ° C and 350 ° C. 8. Method according to claim 1, characterized in that it consists in introducing a plasma containing at least 10% of hydrogen. 9. Method according to claim 8, characterized in that it consists in introducing a plasma containing hydrogen and argon. 10. Method according to claim 1, characterized in that the bed consists of particles of a material chosen in particular from the group consisting of oxides, carbides, nitrides and borides. 11. Method according to claim 10, characterized in that the particles have a catalytic effect. 12. Method according to one of claims 10 or 11, characterized in that the bed also contains a catalyst. 13. Method according to one of the preceding claims, characterized in that the conversion reaction is carried out at a temperature between 500 ° C and 1200 ° C and preferably between 500 ° C and 800 ° C.

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